Such a kind of conventional stopper is constructed such that a stopper member is pushed by a spring force of a spring so that it is fixed by a frictional force. However, the force of a spring is weak. Accordingly, there has been known a stopper which is controlled by air pressure, etc., but this stopper is large in size and complicated in control as well. Here, note that as a similar technique, there is one as described in Japanese Patent Application Laid-Open No. 2009-006458, for example.